Electric Baseboard and Space Heater Sizing Calculator
Electric resistance heat turns every watt into 3.412 BTU/h. Find the room’s heat loss and the heater wattage that replaces it.
Method reviewed October 26, 2026. Version 1.0.0. How it works
This is an estimate. It uses a simplified method and typical assumptions, all listed below. Use it to plan and to check quotes; equipment should be sized from a full room-by-room load calculation (ACCA Manual J) by a qualified professional.
Result
- Heater wattage needed
- 1,110W
- Current at 240 V
- 4.62A
About 1,110 W of electric heat (4.6 A at 240 V) replaces the 3,787 BTU/h the room loses at design conditions.
- Heaters come in fixed sizes; choose the nearest size at or above this figure. Circuit, wire and breaker sizing follow the heater’s instructions and the electrical code — have a licensed electrician install hard-wired heaters.
- Heater size
- 1.11kW
- Room heat loss
- 3,787BTU/h
Show the working
- Design conditions70 °F inside, 0 °F outside (ΔT 70 °F); walls R-11, ceiling R-30, windows U-0.49, 0.6 air changes per hour
- Room heat loss (walls, windows, ceiling, floor and air leakage; no ducts)3,787 BTU/h
- Watts = BTU/h ÷ 3.4123,787 ÷ 3.412 = 1,110 W
- Current = watts ÷ volts1,110 ÷ 240 = 4.6 A
From heat loss to watts
Electric baseboard heaters, wall heaters and plug-in space heaters convert all of their electricity to heat, so the conversion is exact: watts = BTU/h ÷ 3.412. The current is watts ÷ volts.
If you know the room’s design heat loss from a load calculation, enter it directly. Otherwise the tool estimates it from the room’s floor area, exterior wall length, windows, what is above and below, a preset insulation level and your climate — the same component method as our Room-by-Room Load Estimator, without duct losses.
Choosing heaters
Heaters come in fixed sizes, so choose the nearest size at or above the result — or split it between two heaters on long outside walls or under windows. Plug-in space heaters are limited by the outlet circuit they share with other loads; larger rooms usually need hard-wired 240 V heat or several heaters on separate circuits.
Wire size, breaker size and the number of heaters per circuit follow the heater’s instructions and the electrical code. Have a licensed electrician install hard-wired heaters.
Worked example
These are the example values the calculator opens with, and the result the engine calculates for them.
Inputs
- Start from
- Room details (estimate the heat loss)
- Climate
- Zone 5 — cool (e.g. Chicago)
- Room floor area
- 200 ft²
- Ceiling height
- 8 ft
- Exterior wall length
- 25 ft
- Window area
- 30 ft²
- Roof or attic above?
- Yes (top floor)
- Floor
- Over heated space
- Insulation & air sealing
- Average (code-built 1990s–2000s)
- Window type
- Double pane, clear
- Heater voltage
- 240 V
Result
- Heater wattage needed
- 1,110 W
- Heater size
- 1.11 kW
- Current at 240 V
- 4.62 A
- Room heat loss
- 3,787 BTU/h
About 1,110 W of electric heat (4.6 A at 240 V) replaces the 3,787 BTU/h the room loses at design conditions.
Assumptions
- Electric resistance heat turns 100 % of its electricity into heat: watts = BTU/h ÷ 3.412.
- Room estimates use the same component method as the Room-by-Room Load Estimator, with 70 °F inside and no duct losses (baseboard heat has no ducts).
Limitations
- Room estimates rely on preset insulation, window and air-leakage levels; enter a calculated heat loss for a better answer.
- No allowance is added for fast warm-up after a setback.
Frequently asked questions
How many watts per square foot for baseboard heat?
There is no fixed figure: a well-insulated room in a mild climate may need a few watts per square foot, a drafty room with large windows in a cold climate several times more. The room estimate shows why.
Does a 240 V heater use less electricity than a 120 V one?
No. For the same wattage, the energy and the heat are identical; a 240 V heater simply draws half the current.
Is electric resistance heat 100 % efficient?
At the heater, yes — all the electricity becomes heat. A heat pump delivers more heat per kWh, which is why it usually costs less to run.
Sources
- NIST Guide to the SI (SP 811), Appendix B.9 — conversion factors — National Institute of Standards and Technology. 1 ton of refrigeration = 12,000 BTU/h; 1 BTU/h = 0.2930711 W; 1 atm = 101.325 kPa.
- University Physics Volume 2, §9.4 Ohm’s Law — OpenStax, Rice University (CC BY 4.0). V = IR; electrical power P = IV.
- Heat Loss from Buildings — The Engineering ToolBox. Conduction heat loss Q = U·A·ΔT with U = 1/ΣR.
- Calculating heat loss by infiltration (factsheet, 1988) — Washington Energy Extension Service (via AIVC). Infiltration heat loss Q = 0.018 × ACH × volume × ΔT.
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